Distribution and developmental changes in membrane dipeptidase activity were examined in rat and human tissues. The activity to hydrolyze glycyl-D-alanine in rat and human tissues was completely or almost completely inhibited by 5 mM cilastatin, suggesting that the activity was due to membrane dipeptidase and that the contribution of leucine aminopeptidase to the activity was minor. In 8-week-old rats, the activity was high in lung, kidney, pancreas and testis, and in each pooled sample of ileal mucosa, duodenal mucosa, jejunal mucosa and adrenal mucosa. A low activity was found in spleen, liver, serum and heart. The activity in lung, kidney, adrenal and intestinal mucosa increased up to the age of 5 or 8 weeks, while that in pancreas, testis and spleen reached a maximal level at around 3 weeks and declined thereafter. The distribution profile of the enzyme in postmortem tissues of adult humans was similar to that in rat, except for an extremely low activity in lung. The enzyme was also found in serum and urine from healthy volunteers. In urine, the activity was significantly correlated to the creatinine content. No clear dependence of the activity on gender or age was observed in urine and serum.